1. Field of the Invention
This invention relates to enhanced oil recovery processes, whereby aqueous polymer solutions are used to drive oil to a producing well. Stabilization of the viscosity of the polymer solutions is a problem which has plagued such flooding efforts. This invention relates to such stabilization efforts.
2. Description of Related Art
Crude oils are accumulated into geologic traps in the earth, wherein the pores of the rock contain crude oil and connate water. Wells drilled into the geologic traps recover the crude oil by a variety of processes. In what are called "primary" production processes, oil flows to the wellhead at the surface driven by natural pressure or the oil is lifted to the surface by artificial means, such as pumps. In "secondary" production processes, fluids are injected into the oil reservoir through some wells to increase pressure in the oil reservoir or to assist in driving or displacing oil to wells where it can be produced to the surface through other wells. At times, even after a secondary process has been practiced, a "tertiary" recovery process will be employed by further injection of a fluid to increase the amount of oil produced. The fluid injected in either a secondary or tertiary process is often water, aqueous solutions or steam. The crude oils produced vary from oils with a viscosity less than water to oils that are very viscous, even tar-like at ambient temperatures.
Even if the oil is low in viscosity, this process of water displacement of oil leaves large quantities of oil trapped by capillary forces in the pores of the rock. It is well-known in the art to add chemicals such as surface active materials to the water that is injected into oil reservoirs to decrease the capillary forces and to allow more of the oil to be produced. When surface active chemicals are used, there is often a need to drive the chemicals through the reservoir with a following water solution that is made more viscous by the presence of polymers. The polymer solution, or polymer bank as it is often called, is also driven by a fluid, often brine from the very reservoir being produced. It is known in the art that a greater volume of oil-containing rock is contacted by the chemical solutions when they are driven by more viscous driving fluids and, thereby, larger amounts of oil are recovered. A problem arises in many instances, however, because the viscosity of the polymer solution degrades during the time it is in contact with the reservoir rock, particularly rock having acidic sites which react with the polymers. The problem increases in severity as the natural temperature of the reservoir increases.
In oil reservoirs where the crude oil present is highly viscous in its natural state, when water is injected to drive the crude oil to producing wells, the water tends to channel through the viscous oil and leave a large volume of the oil in the rock. Water production will often reach uneconomic amounts before a significant amount of the viscous oil is displaced. Three approaches to increasing the amount of viscous oil produced are well-known in the art: (1) decreasing the viscosity cf the oil by heating it, (2) increasing the viscosity of the water by adding polymers to it, or (3) using a combination of (1) and (2). When the oil in a reservoir is heated, for example, by injection of steam, it is known in the art that injection of viscous water solutions following the steam will often produce additional oil. The reservoir temperature is higher after steam injection and thus the aqueous solutions of polymers are exposed to abnormally high temperatures. The higher temperatures cause the polymers to degrade more rapidly. There is a critical need for methods to allow the polymers employed to maintain their structure and move through the rock, particularly acid rock, with the injected water, so that their benefits are realized in displacing more of the crude oil in the reservoir.
Several types of water-soluble polymers are known in the art to increase the viscosity of aqueous solutions and are used to drive chemicals through a reservoir or to displace viscous oils from a reservoir. Many water-soluble polymers are extremely expensive, which makes their use in a reservoir to recover oil prohibitive unless the price of crude oil at the wellhead is also very high. Attempts to use less expensive water-soluble polymers are thwarted by the problem of rapid degradation of the polymers in the oil reservoir due to contact with the formation rock, with the resultant loss of viscosity. This is particularly true in the reservoir in the Kern River Field in California.
Some of the desirable and relatively inexpensive water-soluble polymers include polyacrylamides, polyacrylates and polymers which are produced by living organisms, called biopolymers, such as polysaccharides, particularly xanthum gum and scleroglucan. Even though effective for a short time, the viscosity of these solutions diminishes and the solutions become ineffective when in contact with reservoir rock for periods of time necessary for their use. Efforts to stabilize them have been wanting. Various attempts have been made to treat reservoir rock so that the viscosity of polymers will not be degraded at high temperatures. Particularly, sodium carbonate or sodium hydroxide have been used, but they have not been proven economically effective. It is an advantage of this invention that a cheap material, urea, will, at relatively low concentrations, tend to neutralize the acidity in the rock and reduce the degradation of polymers in the reservoir.